Vertical cavity surface emitting lasers (hereinafter referred to as “VCSEL's”) have become the dominant light source for optical transmitters used in short-reach local area networks and storage area network applications, in which a multi-mode optical fiber is used for data transmission. VCSEL's are low cost micro-cavity devices with high speed, low drive current and low power dissipation, with desirable beam properties that significantly simplify their optical packaging and testing. In order to extend the application of VCSEL's to optical networks with a longer reach, e.g., in Metropolitan Area Networks that are based on single-mode optical fibers, a long wavelength VCSEL is needed that can emit sufficiently high single mode output power in the 1.3 μm to 1.5 μm wavelength range.
The simultaneous requirement of high power and single mode lasing operation create an inherent contradiction in the VCSEL design. Whereas high power requires a large effective gain volume, single mode operation mandates a smaller active area that is typically less than 5 μm in cross section. This contradiction may be resolved by increasing the longitudinal extent of the gain volume while restricting its lateral area, but in practice this approach is limited by the diffusion lengths of the injected electrical carriers, which limit the thickness of the gain volume. This, along with the stronger temperature dependence of the lasing mode and the gain peak at longer wavelengths, has effectively limited the maximum single mode output power of a long wavelength VCSEL to 1 mW or less before the onset of thermal roll-over.
The use of multiple quantum well stacks arranged in a resonant gain configuration (with the quantum wells located at the anti-nodes) can greatly increase the gain volume and total optical of the VCSEL, but in practice this is limited by carrier diffusion to a single MQW stack. One approach for circumventing the carrier diffusion limit is to electrically cascade successive pn junctions formed by embedding individual gain regions (MQW stacks) between p-doped and n-doped contact layers. The successive pn junctions are electrically “shorted” and thus serially connected by means of Esaki tunnel junctions connecting neighboring p+-doped and n+-doped contact layers. While this approach can result in a higher optical output and differential slope efficiencies that exceed 100%, its principal drawback is the additive nature of the junction voltages, which require the use of high voltage drivers that are not readily available at high modulation speeds.
It would be highly advantageous, therefore, to remedy the foregoing and other deficiencies inherent in the prior art.
Accordingly, it is an object of the present invention to provide a new and improved method of fabricating an electrically pumped long wavelength vertical cavity surface emitting laser.
It is an object of the present invention to provide a new and improved method of fabricating an electrically pumped long wavelength vertical cavity surface emitting laser which operates at lower power.
It is another object of the present invention to provide a new and improved method of fabricating an electrically pumped long wavelength vertical cavity surface emitting laser which has improved light emission properties.
It is another object of the present invention to provide a new and improved method of fabricating an electrically pumped long wavelength vertical cavity surface emitting laser which generates less heat.
It is still another object of the present invention to provide a new method of improving the temperature performance of an electrically pumped long wavelength vertical cavity surface emitting laser which has a reduced temperature dependence.